This invention relates to an open waveguide electromagnetic wave radiator for heating a plasma, and more particularly to an electromagnetic wave radiator whose waveguide maintains a high vacuum during use.
One of the known radiators of this type is a radio frequency (RF) heater which is used in a nuclear fusion reactor.
FIG. 1 schematically shows a nuclear fusion reactor. The reactor comprises a doughnut-shaped vacuum container 12 with coils 16.sub.l -16.sub.n and coils 18.sub.l -18.sub.m are both wound about the container 12. A plasma 14 is produced within the container 12. Neutral beam injectors (NBI) 20.sub.1 to 20.sub.6 are installed around the container 12 and coupled thereto. They inject energetic atoms into the container 12, thus heating the plasma to a predetermined temperature.
A RF heater 22 extends through the gap between two of the NBIs and is connected to the vacuum container 12. It comprises a waveguide 24 opening at one end, a radiator 25, a protective wall 26 and a power feed line 28. The radiator 25 is placed within the vacuum container 12. The open end of the waveguide 24 communicates with the radiator 25. The waveguide 24 is wrapped by the protective wall 26. Electromagnetic waves of cyclotron frequency for exciting plasma ions are supplied to the waveguide 24 from a RF power source (not shown) through the power feed line 28. An evacuation means (not shown) is coupled to the vacuum container 12. The evacuation means draws air out of the vacuum container 12, thus maintaining a vacuum in not only the container 12, but also the waveguide 24.
The transmissive power of electromagnetic waves through the waveguide 24 depends on the vacuum in the waveguide 24. The higher the vacuum, the higher the power. The waveguide 24 is made of metal. Various gases may be released from the metal and may fill up the waveguide 24. If this happens, the vacuum will be degraded. Gases are drawn from the waveguide 24 only through the aperture of the radiator 25. Unless this evacuation catches up with the release of gases, the transmissive power of electromagnetic waves will be lowered. The reduction in transmissive power does not matter when a relatively small amount of power is supplied to the waveguide 24 and the waveguide 24 is thus relatively short. When a great amount of power is supplied to the waveguide 24 and the waveguide 24 must inevitably be long, the transmissive power of electromagnetic waves will be reduced, thus causing problems.
To make matter worse, the gases from the waveguide 24 may flow into the vacuum container 12, thus lowering the vacuum within the container 12. This leads to a reduction in the heating efficiency of plasma. In order to prevent this, a large, powerful evacuation means must be installed.